/Engine/Source/Runtime/MovieScene/Public/EntitySystem/MovieSceneEntityManager.h
Set up an entity to be a child of another. Child entities will only be cleaned up if their parents are marked Unlink, and TagOrphanedChildren is called
void AddChild ( [FMovieSceneEntityID](API\Runtime\MovieScene\EntitySystem\FMovieSceneEntityID) ParentID, [FMovieSceneEntityID](API\Runtime\MovieScene\EntitySystem\FMovieSceneEntityID) ChildID )